What is the role of lidocaine or phenytoin in tricyclic antidepressant-induced cardiotoxicity?

INTRODUCTION Tricyclic antidepressant (TCA) poisoning is a relatively common occurrence and remains a significant cause of mortality and morbidity. Deaths from TCA toxicity are typically due to cardiovascular events such as arrhythmias and hypotension. Cardiovascular toxicity may be multifactorial. However, the primary mechanism is a TCA-induced membrane-depressant or "quinidine-like" effect on the myocardium resulting in slowing down of phase 0 depolarization of the cardiac action potential and subsequent impairment of conduction through the His-Purkinje system and myocardium. This effect is manifest as QRS prolongation on the EKG, atrioventricular (AV) block, and impairment in automaticity leading to hypotension and ventricular dysrhythmia. Primary treatment strategies include sodium bicarbonate, hypertonic saline, and correction of any conditions that may aggravate this toxicity such as acidosis, hyperthermia, and hypotension. In cases of severe TCA toxicity, administration of sodium bicarbonate may be insufficient to correct the cardiac conduction defects. Use of lidocaine or phenytoin, both Vaughan Williams Class IB antiarrhythmic agents, has been reported as an effective adjunctive therapy in cases of severe cardiotoxicity. METHODS Thirty articles of interest were identified by searching PubMed, abstracts from meetings, and the reference sections of related primary and review articles and toxicological texts. ROLE OF LIDOCAINE AND PHENYTOIN: Lidocaine and phenytoin also cause sodium channel blockade, but unlike Class IA or IC agents do not depress phase 0 depolarization in healthy cardiac tissue. Lidocaine and phenytoin dissociate relatively quickly from cardiac sodium channels. Sodium channels have faster recovery times after exposure to lidocaine (1-2 s) and phenytoin (0.71 s), than with some TCAs such as amitriptyline (13.6 s), but not others (e.g., imipramine at 1.6 s). In experimental models of amitriptyline poisoning, lidocaine co-administration resulted in decreased sodium channel blockade compared to amitriptyline alone. This correlated with clinical improvement, including normalization of QRS interval, improved hypotension, and decreased mortality. It is postulated that lidocaine's rapid binding to the sodium channel may directly displace slower acting agents from the channel, leaving more channels unbound, and therefore be able to facilitate cardiac conduction. Phenytoin may act through a similar mechanism as lidocaine, although experimental studies suggest that it does not compete directly for the same sodium channel binding site as TCAs. Allosteric modulation of the TCA binding site may occur in the setting of phenytoin use. The evidence for using phenytoin in treating TCA-induced sodium channel blockade is less convincing than that for lidocaine. Human trials are limited to case series and, in most human exposures in which there appeared to be efficacy, the toxicity was not severe. CONCLUSIONS Although there appears to be more evidence for the use of lidocaine than phenytoin as adjunctive treatment for TCA-associated cardiotoxicity, specific clinical indications and dosing recommendations remain to be defined. We recommend the use of lidocaine in cases in which cardiotoxicity (arrhythmias, hypotension) is refractory to treatment with sodium bicarbonate or hypertonic saline, or in which physiological derangement (e.g., severe alkalosis or hypernatremia) limits effective use of these primary strategies.